Summoning Karstaag

Summoning Karstaag is an unmarked quest in Skyrim's Dragonborn DLC. Karstaag is a secret boss, much like The Ebony Warrior. The quest involves finding the skull of Karstaag and reuniting it with the rest of his skeleton in Castle Karstaag Ruins. Doing so will summon his ghostly form to fight you. Defeating him will grant you the ability to summon him three times (outdoors only).

Begin by heading into Glacial Cave. It is a small cave on the northern end of Solstheim not far from Castle Karstaag. Towards the back of the cave, guarded by a few Rieklings, is the Skull of Karstaag. Take it and head to Castle Karstaag. The main gate is master locked, but can be picked. Alternatively, you can enter Castle Karstaag Caverns and work your way up through the tribe or Rieklings into the castle courtyard, bypassing the main gate.

Once inside, defeat the few Rieklings and approach the throne. Interact with the throne to place the skull with the rest of the body, and Karstaag will be summoned. Karstaag is a ghostly frost giant, and uses many similar attacks that a Frost Giant would, only to greater effect. His stomp is so powerful it mimics Unrelenting Force and can throw the player across the room. He also has a permanent frost cloak, and a blizzard that will cast constant frost damage throughout the battle. Karstaag will also summon three Ice Wraiths at a time to aid him.

Once you have defeated him you will gain the power to summon Karstaag, though it can only be used three times, and outdoors only.

There is a bug that can happen if you are thrown by Karstaag's stomp and into the nearby Castle Karstaag Caverns entrance. Upon loading into the caverns your character will be stuck on the ground, but only in third person view. While you can move around in a hilarious fashion, you will be unable to attack or cast spells in third person view. Because of this it is reccomended to save before summoning Karstaag.

Spirit of Karstaag has the same body as frost giants found in forgotten valley (Dawnguard). He even has the same giant's club